South Atlantic upper-ocean flow redistributions and their connection to overturning in the North Atlantic

The upper cell of the Atlantic meridional overturning circulation (AMOC) comprises a deep lower limb exporting North Atlantic deepwater (NADW) southward, fed by a shallow upper limb carrying interocean waters northward from the southern South Atlantic and thereby enabling continued NADW production. On decadal and longer timescales, high-latitude density anomalies affecting the production of NADW are expected to generate coherent overturning transport changes across latitudes in the Atlantic basin. This process calls for compensating effects in upstream components of the northward AMOC upper limb, wherein upper-ocean meridional transports are adjusted according to flow continuity principles, tending to even out mass imbalances.

Besides the South Atlantic functioning as a mediator of interocean exchanges that are crucial for driving and maintaining the AMOC, the subtropical South Atlantic is the only region where the total northward component of the AMOC upper limb is subjected to ocean interior dynamics along its interhemispheric trajectory — before being settled over the Atlantic western boundary, a narrow crossroad along which it proceeds up to the subpolar North Atlantic by delineating the American coastline. This results from the AMOC upper limb being incorporated into the anticyclonic South Atlantic subtropical gyre (SASG) at its origins. By crossing the subtropical South Atlantic and meeting the South American coast, the AMOC upper limb is ultimately decoupled from the SASG and placed over the South Atlantic western boundary — as the westward flow along the SASG northern boundary bifurcates meridionally, redistributing waters between these two major large-scale circulation systems.

This work aims to demonstrate that the singular dynamic setting of the South Atlantic general circulation, compared to its North Atlantic counterpart, makes it more vulnerable to large AMOC changes. The findings from two recent studies based on transient deglacial simulation results will be discussed, which suggest that the upper-ocean flow redistributions taking place over the South Atlantic western boundary are highly responsive to AMOC changes under pre-industrial paleocean dynamics, and thus have the potential to provide future insights into the degree and timescales over which overturning in the North Atlantic impacts adjacent ocean basins and vice versa.

This model-based perspective elucidates the link between the South Atlantic western boundary current system and the AMOC through the establishment of meridional connectivity of AMOC variability. By definition, the steady-state AMOC system is meridionally coherent. Questions that naturally arise are: How is AMOC meridional coherence transiently modulated? And how will this modulation process evolve under modern climate change conditions? These questions are particularly relevant for upcoming research efforts dedicated to understanding how the South Atlantic circulation is to be shaped by climate change.